The present invention relates to a method for increasing the cycle stability of a latent heat accumulator medium consisting essentially of a Glauber's salt melt. When using dynamic latent heat accumulators employing Glauber's salt as the latent heat accumulator medium, a fluid heat exchange medium is passed directly through the Glauber's salt melt to exchange heat. After passing through the accumulator medium, the heat exchange medium separates from the Glauber's salt melt. Low-viscosity oils are normally used as heat exchange medium.
Latent heat accumulators are generally known in which a Glauber's salt melt is used as the accumulator medium. The salt melt can be produced, for example, from technical sodium sulfate (Na.sub.2 SO.sub.4) and water wherein 44.1% by weight Na.sub.2 SO.sub.4, corresponding to 100% Na.sub.2 SO.sub.4.10 H.sub.2 O, is used. It is also possible to employ the composition of the solution which is saturated at 32.degree. C. wherein 33.2% by weight Na.sub.2 SO.sub.4, corresponding to 75.4% Na.sub.2 SO.sub.4.10 H.sub.2 O, is used.
However, Glauber's salt itself is not normally used for preparation of the accumulator medium since its practical application as a commercial product is limited owing to its water of crystallization content of approximately 56% by weight, the high transport costs thereby incurred, its low melting point of approximately 32.degree. C., and its tendency to effloresce when stored for a long time. Furthermore, owing to the incongruent behaviour of Glauber's salt, it forms a two-phase system during melting, at a temperature of 32.4.degree. C. for the very pure compound, consisting of a saturated solution with the aforementioned composition and solid sodium sulfate.
The initial salts employed in such latent heat accumulators are, as a rule for reasons of cost, technical products of varying purity. Moreover, during the life-span of the Glauber's salt accumulator medium, further contaminations are introduced into the melt, for example by the constant contact of the Glauber's salt melt with the container and the conduit pipes, as well as by the heat exchange circuit communicating with the accumulator container. In particular, corrosion products of metals coming into contact with the accumulator medium form in the salt. These contaminations usually lead, by chemical reactions with the accumulator medium and by other influences, to a loss of capacity and a reduction in the operating temperature of the accumulator, whereby the cycle stability of the accumulator medium is greatly reduced.
Experimental tests have shown that the pH value of the Galuber's salt melt produced from technical sodium sulfate is in the range of from 4.5 to 6.5. The melt is plainly cloudy and contains hydroxides in differing amounts, mainly brown iron hydroxide. If the pH value of the Glauber's salt melt is displaced into the alkali range for example by adding alkali hydroxides or similar substances, a protolysis reaction with water occurs and a quantitative precipitation of the heavy metals as metal hydroxides having a low solubility will result.
In order to prevent any supercooling from occurring in the Glauber's salt latent heat accumulator medium during the discharge of heat, it is well known in the art to add inoculating nuclei to the Glauber's salt melt, usually in the form of borax (Na.sub.2 B.sub.4 O.sub.7.10 H.sub.2 O), for enhancing the nucleation of the salt. However, the addition of borax also displaces the pH value into the alkali range, the pH value of the resultant Glauber's salt melt being between 8 and 9.5, depending on the concentration of borax which is added, thereby precipitating impurities. When the salts are employed in dynamic latent heat accumulators with direct heat exchange, the precipitated metal hydroxides are deposited on the interface of the melt and the heat exchange oil and pass into the heat exchange oil in varying amounts, thereby causing considerable problems in the circulation system for the heat exchange oil.
Due to the aforementioned adverse influences, which have an increasingly stronger effect during the life-span of the Glauber's salt melt, the Glauber's salt accumulator medium could not be operated trouble-free over a prolonged period if time.